This invention relates to a method for in situ determination of the concrete strength in concrete structures or parts thereof.
The concrete industry has for a long time requested a rapid and practical tool to evaluate the rate of hardening in a concrete structure itself, both in order to determine when forms can be removed safely, and to establish the most favourable curing and production methods.
Today several methods are available for determination of the concrete strength in a structure:
1. Non-destructive methods (recoil hamer, ball indentor, Windsor Probe Test) measure the elastic properties in the surface zone of the concrete. These methods suffer from the drawback that they can only determine properties near the surface, and consequently they are strongly influenced by local disturbances (impurities and moisture content at the surface), and the fact that the elastic properties being measured have low correlation to conventional strength parameters.
2. Destructive methods are based on removal of specimens from the structure (by drilling out or by pre-set forms) and subsequent conventional testing in testing machines. All these methods are complicated, time-consuming and costly. Besides, drilling techniques cannot be used at low strength levels as the specimens will be damaged by the drilling load.
3. Semi-destructive methods are based on elements such as bolts or lugs embedded in the casting and measuring the force necessary to pull these elements out of the casting. These methods have the shortcomings that the stress condition and rupture zone at the moment of rupture are weakly defined. Further they are relatively complicated and time-consuming.
Up till now, in order to determine the strength properties in local zones differently spaced from the surface of the concrete, it has been necessary to remove specimens (by drilling, chiseling, etc.) which, after forming representative test pieces for the respective zones may be subjected to conventional strength testing.